Apparatus for casting metal filaments through an aerosol atmosphere

ABSTRACT

Metallic filaments are produced by quenching a stream of molten metal extruded from a melt with an aerosol such as a suspension of finely divided fluid particles in air. The aerosol is produced by atomizers which provide circulation of the aerosol along and across the stream of molten metal.

United st tes Patent Robert B. Pond Westminster, Md. 757,257

Sept. 4, 1968 Aug. 31 1971 The Battelle Development Corporation Columbus. Ohio inventor Appl. No. Filed Patented Assignee APPARATUS FOR CASTING METAL l-ILAMENTS THROUGH AN AEROSOL ATMOSPHERE 3 Chilns, 1 Drawing F.

- 164/82. 264/ 176 F, 264/205 lot. 82211 11/12 Field olSearch 164/81, 82,

[56] References Cited UNITED STATES PATENTS 3,366,721 1/1968 Burdge et al 264/176 (F) X 3.415.922 12/1968 Carter et al... 264/176 (F) 3,461,943 8/ 1969 Schile, 164/89 FOREIGN PATENTS 702,687 y l/1954 Great Britain 164/283 Primary Examiner-R. Spencer Annear Anomey- Larson, Taylor & Hinds ABSTRACT: Metallic filaments are produced by quenching a stream of molten metal extruded from a melt with an aerosol such as a suspension of finely divided fluid particles in air. The aerosol is produced by atomizers which provide circulation of the aerosol along and across the stream of molten metal.

1 mi l IIIH PATENTEUAUG31I971 3,602,291

INVENT OR ATTORNEYS ROBERT B. POND APPARATUS son CASTING METAL FILAMENTS THROUGH AN AEROSOL ATMOSPHERE FIELD OF THE INVENTION The present invention relates to a method and apparatus for free casting'metal fibers or filaments and more particularly to the solidification of extruded molten metal streams into such fibers or filaments during a free casting process. 1

BACKGROUND OF THE INVENTION The continuous casting of metal fibersas free castings, that is, castings without molds, wherein the surface tension of the molten metal serves to contain the metal until solidified, has more recently come to be known as jet spinning. In an example of such casting a melt of the metal is extruded. through an orifice to produce a stream-of molten metal which passes through a quiescent quenching medium. In considering the manufacture of wires or fibers of symmetrical cross section In order to best understand these limitationsone may examine the jet spinning of a metal of extremely high surface tension with normal values of specific heat andlatent heat of fusion at various ejection velocities. An exemplary. metal of this type is cast iron. Although there is some uncertainty about the precise value forthe surface tension of cast iron the ap parent surface tension is generally accepted to be above I000 dynes/cm. in air. If molten cast iron (with l superheat) is ejected in a continuous manner from a smooth, round orifice at velocities less than 5 feet per second, the Rayleigh pinch off point will be within 6 inches of the orifice. This meansthat the heat transfer rate between the metal and the quenching'medium will have to be sufficiently large and the temperature gradient sufficiently steep so that solidification of the. molten stream can be'ensured to occur within the-6 inch interval, that is, before pinch off for fiber fabrication. It is noted that if the ejection velocity is approximately 20 feet persecond the distance to the pinch off point will be increased to approximately 24 inches. The increase in distance to the pinch off point is greaterat higher ejection velocities and at an ejection velocity. of approximately 40 feet per second the pinch off. point can be made to exist at at least 48 inches from the orifice. For the conditions set forth, in a quiescent atmosphere-of air at relative humidities below 50 percent, solidification of astream of molten cast iron will not occur until the stream has traveled approximately 2.5 feet at five feet per'second, 10 feet at 20 feet per second, or 20 feet at 40feet per second. It will be appreciated that any product produced by such-a'process would be in the formof powder or shot.

In order to enable more rapid solidification of the stream or jet of metal a number of variations in the. stated conditions may be tested. For example, the Nusselt number, a dimensionless number related to the heat transfer coefficient, can be increased by causing a transverse fiow of air across the axis of the filament or fiber to be produced. Such a transverse flow of air can be produced by ejecting the molten metal at an angle to the direction of the gravitational force and letting the biased stream free fall, or by installing a fan at a suitable location along the axis of the stream so thata gentle transverse solidification of the metal can be effected. If, on. the other hand, the metal does riot form a surface compound in thatmedium but. rather provides adsorbed films which themselves serve to decrease the surface tension by an amount sufficient to enable the pinch 'off point to be extended beyond the solidification'point, the metal can still be properly solidified. It

is noted-that a third type of medium may be envisioned wherein an adsorbed film is produced or/and a compound. formed wherein the surface tension of the metal is not drasti I cally reduced or a conduit for the molten metal provided but whereinthe solidification process is nonetheless facilitated because the surface reaction is endothermic and thus provides the effect of an increased quench rate. It is noted that the compound formed underthe circumstances may even be soluble in the molten metal. v

' A yet further improvement in the process may be produced when the quench characteristics of the quenching medium used is improved. Inspection of the literature on this subject will reveal that the relative conductivities of various gases only change by' factors-whereas what is required is a change by at least an order of magnitude and thus the substitution of one gaseous quench medium for another will .7 not generally produce any substantial improvement in the quenching rate. Certain liquid quenching media such as refrigerated saline solutions have been found to provide severe quenching of l I the type desired but the use of such media has at least one serious drawback. Specifically, the inertial effect of a liquid such as water is so great that a rapidly moving stream of molten metal, which has no shear strength, tends. to blow apart upon striking the surface of the liquid..Thus, for. this reason theuse ofquenching solutions of this type is not generally feasible.

SUMMARY OF THE INVENTION In accordance with the present invention an improved quenching. medium for use in jet spinning is provided whereby quenching. of a-streamof molten metal is significantly accelerated'so that completed metallic filaments, in contrast to powder orshot, may be formed. The improved quenching medium comprises an aerosol of a vaporizable liquid or solid such as, for example, a-fog of finely divided, uniformly flow of gas is provided. Another approach is to provide a.

pound which is nonsoluble in the metal and which has a stability greater than that of the stream itself, the surface compound will serve as a conduit to preserve steam' shape until dispersed aqueous particles. Theuse of an aerosol form of a substance permits the utilization of the desirable quenching characteristics of such a substance while at the same time eliminates the undesirable inertial effects produced thereby which tend to shatter the stream of molten metal.

To explain the operation of such an aerosol it is noted that if theaerosol is quiescent, thestream of molten metal in contacting a particle or droplet will cause the droplet instantly to evaporate byelevatingthe temperature thereof. Evaporation of the droplet will lower the environmental temperature by thelatent heat of vaporization of the substance forming the droplet. The extent to which a given volume can be refrigerated is generally a function of the number of droplets evaporated per unit time. in that volume. Therefore, the

refrigerating effect should be directly proportional to the number of particles in the volume. This particle density will, of course, be a function of the kind and extent of packing of the particles. It will be appreciated that the molten stream does not have to directly contact a droplet in order to promote refrigeration in view of the fact that radiation from the surface of the-molten stream can also warm the volume immediately adjacent to each droplet. The distance over which such radiation is effective is also a function of the packing of the particles. v

In accordance with a presently preferred embodiment of the invention an arrangement is provided for recirculating the aerosol in a continuous fiow path. In accordance with an important feature of the invention the flow path includes cross flow, that is, flow across the stream of molten metal, which increases the turbulence of the medium andconsequently increases the coefficient-of heat' transfer. In a specific embodiment,'the.arrangement includes a main pipe or tube through which the stream of molten metal' passes and a second pipe laterally ofiset therefrom and connected thereto at upper and lower positions along the length thereof by means of upper and lower generally horizontal connecting pipes. A first atomizer positioned to produce an aerosol spray along the upper connecting pipe and across the stream of molten metal anda second atomizer positioned to produce an aerosol spray along the lower connecting pipe and across the stream of mo]- ten. metal cooperate to provide continuous circulation of the aerosol. The atomizers are arranged such that a significant portion of the aerosol flow in the main pipe takes place in a direction parallel to the movement of the stream of molten metal, that is, downwardly.

Other features and advantages of the present invention will be set forth in or apparent from the description of the preferred embodiment of the invention found hereinbelow.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, a furnace includes a holding tube 12 and an orifice 14 through which is extruded a continuous'stream or jet of molten metal generally denoted 16. Furnace 10 may be of any suitable conventional construction and further description thereof is deemed unnecessary. The extruded metal 16 passes through a quenching arrangement generally denoted l8 and described in detail hereinbelow', to a container 20 which contains a liquid 22 such as water.

Quenching arrangement 18 comprises a main pipe or tube 24 through which the stream passes and a second laterally off set, generally parallel pipe 26. An upper connecting pipe 28 joins the upper end of pipe 26 toan upper opening along the length of main pipe 24 while a lower connecting pipe 30 joins the lower end of pipe 26 to a lower opening along the length of main pipe 24. The aerosol quenching medium of the present invention is created by first and second atomizers 32 and 34. Atomizers 32 and 34 are individually connected to a suitable source of quenching liquid to be atomized (not shown) and to a suitable source of a gas under pressure (not shown). The quenching liquid may for example be water and the pres surized gas may be air although as is set forthhereinbelow a number of other aerosols may beem'ployed. Although it will be appreciated that a number of different atomizing arrangements may be utilized an atomizer manufactured by the Spraying Systems Company of Belville, 111., which produces a droplet or particle size from 10 to 50 microns has been found to be satisfactory.

As shown in the drawing, the atomizer 32 is positioned at the junction between upper connecting pipe 28 and offset pipe 26 opposite the upper opening in main pipe 24 while atomizer 34 is positioned in the wall of main pipe 24 opposite the opening communicating with lower connecting pipe 30. Atomizer 32 produces a spray which travels along connecting pipe 28 and crosses the path of the molten metal stream 16. The cross flow created by atomizer 32 introduces turbulence into the aerosol and thereby increases the coefficient of heat transfer in that area. Similarly, atomizer 34 by injectinga spray across stream 16 provides a localized increase in heat transfer by increasing the turbulence in that area. The nozzle opening in atomizer 32 is preferably such that a relatively fine'spray is produced whereas the size of the nozzle opening of atomizer 34, which isof course positioned at a point much further on in the quenching process, is preferably such that a coarser spray is produced. Atomizers 32 and 34 cooperate to produce a Pinwheel" effect such. that the aerosol is accelerated in the area of the atomizer. sprays and continuous circulation of the aerosol is effected.'.As is indicated by the arrows in the drawing the operation :of atomizers 32 and 34 create a flow of aerosol in main pipe24 in the same direction as the flow of extruded stream 16. Although one might speculate that countercurrent flow of the aerosol would be more thermally recuperative than current flow such current flow tends to cause escape of the aerosol at or near orifice 14 thereby causing a refrigerating effect at a locality where maintenance of the temperature above the solidification point is required to effect ejection of the molten metal Container 20 positioned below main pipe 24 is optional but provides a number of advantages. For example,,when a fog, that is, a dispersion of aqueous particles, is used as the aerosol there will be condensation on the sides of the main pipe 24 and water will drip down onto the floor in the absence of container 20. Further,- the liquid in container 20 can serve to complete quenchingwhere only an outer shell or conduit has been formed before stream 16 reaches the liquid 22. In addition, the-presence of container 20 and liquid 22 at one end of main pipe 24 has been found to aid in'controlling the circulation of the aerosol.

As set forth hereinabove the aerosol acts to produce quenching of stream 16 either through contact of adroplet with the stream to cause evaporation of the droplet and consequent lowering of the environmental temperature by the latent heat of vaporization or through a similar vaporization caused by radiation from the molten stream 16. If it is presumed that the radiation effect can be felt over a distance of one-half inch then thequench rate per unit column length per unit time for a 1-inch diameter column will be increased by three orders of magnitude (presuming ideal cubic packing). It is noted that where a molten stream passes through a quiescent aerosol chamber the stream will be subject to an increased quench rate along the entire length thereof and although this is also true of the quenching arrangement of the present invention the cross flow created by atomizers 32 and 34 ensures that the quench rate will be accelerated within the locality of the atomizers prior to the Rayleigh pinch off point.

Aerosols or fogs may be produced in a number of ways such as by refrigerating aqueous supersaturated gas, through sudden changes of pressure or by atomization as described hereinabove. Where continuous volumes of an aerosol are necessary and a high evaporation rate is desired the use of atomizers to produce the aerosol is to be preferred.

As set forth hereinabove the quenching aerosol utilized is not necessarily an aqueous dispersion and use can be made of any material which can be produced in an aerosol form. If, for example, water serves to increase the surface tension of the metal then the advantage of the high quench rate produced by water is partially offset because of the earlier pinch ofi" caused by the increase in surface tension. Thus certain materials may have specific advantages depending on the variables of the process. Materials other than water which can be produced as aerosols include such nonfiammable materials as trichloroethylene and Freon l2. Flammable materials such as carbon-disulfide and oils can also be used as long as the carrier gas for the aerosol is nonoxidizing. It is noted that even plastics such as organic polymeric resins can be produced as aerosols.

With certain materials it is possible, depending on the material being spun" to take advantage of more than one of the properties discussed above. For example, if an organic polymeric smoke is used as the quench media the latent heat of the solution of the resin will cause an increased quench rate and at the same time the resin will film the molten jet of metal to provide a stabilizing conduit as discussed hereinabove. If an organic solvent is used the latent heat of vaporization thereof will cause an increased quench rate and within the vapor the surface tension of the metal may, depending on the materials involved, be reduced thereby extending the pinch off point or affording a longer time for solidification of the metal stream into a filament or wire to be effected.

It will be understood by those skilled in the art that the embodiment of the invention shown and described hereinabove is subject to other modifications without departing from the scope and spirit of the invention. Accordingly, it should be understood that the invention is not limited by the exemplary embodiment shown and described but rather by the subjoined claims.

1. In an apparatus for producing metallic filaments wherein a melt of molten metal is extruded to form a molten stream, the improvement comprising means for causing solidification of the stream of molten metal comprising an environment containing an aerosol of a vaporizable substance, means for providing circulation of said aerosol in a continuous path and directing means for causing flow of said aerosol across said stream of molten metal to increase the heat transfer coefficient of said stream, said means for providing circulation of said aerosol being positioned beneath the portion of the apparatus from which the stream of molten metal is extruded and the path defined by said circulation providing means including first and second generally vertical portions interconnected by upper and lower generally horizontal portions, and

said directing means-comprising first and second atomizing nozzles, said first atomizing nozzle producing an aerosol spray along said upper horizontal portion of said path and across said stream of molten metal in a first direction and said second atomizing nozzle producing a second aerosol spray across said v stream of molten metal and along a lower horizontal portion 

1. In an apparatus for producing metallic filaments wherein a melt of molten metal is extruded to form a molten stream, the improvement comprising means for causing solidification of the stream of molten metal comprising an environment containing an aerosol of a vaporizable substance, means for providing circulation of said aerosol in a continuous path and directing means for causing flow of said aerosol across said stream of molten metal to increase the heat transfer coefficient of said stream, said means for providing circulation of said aerosol being positioned beneath the portion of the apparatus from which the stream of molten metal is extruded and the path defined by said circulation providing means including first and second generally vertical portions interconnected by upper and lower generally horizontal portions, and said directing means comprising first and second atomizing nozzles, said first atomizing nozzle producing an aerosol spray along said upper horizontal portion of said path and across said stream of molten metal in a first direction and said second atomizing nozzle producing a second aerosol spray across said stream of molten metal and along a lower horizontal portion of said path in a second, opposite direction, said first and second atomizing nozzles cooperating to provide continuous circulation of said aerosol.
 2. An apparatus as claimed in claim 1 wherein said aerosol spray produced by said second nozzle is relatively coarser than the aerosol spray produced by said first nozzle.
 3. Apparatus as claimed in claim 1 further comprising means containing a liquid for completing the solidification of filaments not completely solidified during the passage thereof through said aerosol. 